The Application relates to platelet-shaped colored pigments in which a platelet-shaped substrate is covered with a layer containing iron oxide.
Colored platelet-shaped pigments containing iron oxide are known per se. In particular, layers of Fe.sub.2 O.sub.3, if appropriate together with other metal oxides, such as, for example, TiO.sub.2, are applied to platelet-shaped substrates, in particular mica. However, it is furthermore also known from German Offenlegungsschrift 2,313,331, which corresponds to U.S. Pat. No. 3,926,659, that layers containing iron(II) oxide can be deposited, uniform layers of magnetite on mica or on mica coated with metal oxides being produced by precipitation from iron-(II) salt solutions in the presence of an oxidizing agent. As explained in the third paragraph on page 7 of German Offenlegungsschrift 2,313,331, these are rough layers which show no interference colors and therefore also have no nacreous luster. Only if mica/metal oxide interference pigments are used as a substrate for a magnetite coating can this interference color still shine through in the case of very thin magnetite layers, but the magnetite layer itself is not capable of producing interference colors of thin platelets.
The same applies to the black pigments described in Example 8 of European Patent Application 0,077,959, which corresponds to U.S. Pat. No. 4,435,220, which are prepared analogously to the process of German Offenlegungsschrift 2,313,331, except that the precipitation of the magnetite is carried out in the presence of an alkaline earth metal salt with the aim of improved heat stability.
Although these known magnetite pigments can readily be used as platelet-shaped black pigments for many purposes, they cannot be referred to as nacreous pigments in the actual sense because of the lack of interference ability and a lack of luster.